What Causes Day and Night?

For most of here on planet Earth, sunrise, sunset, and the cycle of day and night (aka. the diurnal cycle) are just simple facts of life. As a result of seasonal changes that happen with every passing year, the length of day and night can vary – and be either longer or shorter – by just a few hours. But in some regions of the world (i.e. the poles) the Sun does not set during certain times of the year. And there are also seasonal periods where a single night can last many days. Naturally, this gives rise to certain questions. Namely, what causes the cycle of day and night, and why don’t all places on the planet experience the same patterns? As with many other seasonal experiences, the answer has to do with two facts: One, the Earth rotates on its axis as it orbits the Sun. And two, the fact that Earth’s axis is tilted.

Earth’s Rotation:

Earth’s rotation occurs from west to east, which is why the Sun always appears to be rising on the eastern horizon and setting on the western. If you could view the Earth from above, looking down at the northern polar region, the planet would appear to be rotating counter-clockwise. However, viewed from the southern polar region, it appears to be rotating clockwise.

The Earth rotates once in about 24 hours with respect to the Sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars. What’s more, its central axis is aligned with two stars. The northern axis points outward to Polaris, hence why it is called “the North Star”, while its southern axis points to Sigma Octantis.

Axial Tilt:

As already noted, due to the Earth’s axial tilt (or obliquity), day and night are not evenly divided. If the Earth’s axis were perpendicular to its orbital plane around the Sun, all places on Earth would experience equal amounts of day and night (i.e. 12 hours of day and night, respectively) every day during the year and there would be no seasonal variability.
Instead, at any given time of the year, one hemisphere is pointed slightly more towards the Sun, leaving the other pointed away. During this time, one hemisphere will be experiencing warmer temperatures and longer days while the other will experience colder temperatures and longer nights.

Seasonal Changes:

Of course, since the Earth is rotating around the Sun and not just on its axis, this process is reversed during the course of a year. Every six months, the Earth undergoes a half orbit and changes positions to the other side of the Sun, allowing the other hemisphere to experience longer days and warmer temperatures.

rendition of the Earth’s rotation and the precession of the Equinoxes. Credit: NASA

Consequently, in extreme places like the North and South pole, daylight or nighttime can last for days. Those times of the year when the northern and southern hemispheres experience their longest days and nights are called solstices, which occur twice a year for the northern and southern hemispheres.
The Summer Solstice takes place between June 20th and 22nd in the northern hemisphere and between December 20th and 23rd each year in the southern hemisphere. The Winter Solstice occurs at the same time but in reverse – between Dec. 20th and 23rd for the northern hemisphere and June 20th and 22nd for the southern hemisphere.
According to NOAA, around the Winter Solstice at the North Pole there will be no sunlight or even twilight beginning in early October, and the darkness lasts until the beginning of dawn in early March. Conversely, around the Summer Solstice, the North Pole stays in full sunlight all day long throughout the entire summer (unless there are clouds). After the Summer Solstice, the sun starts to sink towards the horizon.
Another common feature in the cycle of day and night is the visibility of the Moon, the stars, and other celestial bodies. Technically, we don’t always see the Moon at night. On certain days, when the Moon is well-positioned between the Earth and the Sun, it is visible during the daytime. However, the stars and other planets of our Solar System are only visible at night after the Sun has fully set.

Night Sky”. On a clear night, the stars and the glowing band of the Milky Way Galaxy are generally visible. Credit: Sam Crimmin

The reason for this is because the light of these objects is too faint to be seen during daylight hours. The Sun, being the closest star to us and the most radiant object visible from Earth, naturally obscures them when it is overhead. However, with the Earth tilted away from the Sun, we are able to see the Moon radiating the Sun’s light more clearly, and the stars light is detectable.
On an especially clear night, and assuming light pollution is not a major factor, the glowing band of the Milky Way and other clouds of dust and gas may also be visible in the night sky. These objects are more distant than the stars in our vicinity of the Galaxy, and therefore have less luminosity and are more difficult to see.
Another interesting thing about the cycle of day and night is that it is getting slower with time. This is due to the tidal effects the Moon has on Earth’s rotation, which is making days longer (but only marginally). According to atomic clocks around the world, the modern day is about 1.7 milliseconds longer than it was a century ago – a change which may require the addition of more leap seconds in the future.
We have many interesting articles on Earth’s Rotation here at Universe Today. To learn more about solstices here in Universe Today, be sure to check out our articles on the Shortest Day of the Year and the Summer Solstice. Reference Link

What Causes the Day/Night Cycle on Earth?

By Serm Murmson; Updated March 09, 2018

The change between day and night is caused by the rotation of the Earth on its axis. If the Earth did not rotate as it does, the day/night cycle would be very different or possibly even nonexistent. The changing lengths of days and nights depends on where you are on Earth and the time of year. Also, daylight hours are affected by the tilt of the Earth's axis and its path around the sun.

Length of a Rotation

A solar day, 24 hours, is the time it takes for the Earth to rotate exactly once so that the sun appears at the same place in the sky the next day. However, the Earth is also moving around the sun, and this movement makes measuring the day somewhat complicated. The actual time of one Earth rotation is a little shorter – about 23 hours and 56 minutes. Astronomers discovered this by observing the time it took for a star to appear in the same place in the sky the following day, and they called this a sidereal day.

Longer and Shorter Days

Although a solar day is 24 hours, not every day has 12 hours of daylight and 12 hours of night. Daytime is shorter in winter than in summer. This is because the Earth’s imaginary axis isn’t straight up and down, it is tilted 23.5 degrees. As the Earth moves around the sun during a year, the northern half of the Earth is tilted towards the sun in the summer, making daytime longer than night. In winter, this reverses; the earth tilts away from the sun and nighttime becomes longer. In the spring and fall, the tilt is neither toward or away from the sun but somewhere between, so day and night are more the same at these times of the year.

The Solstices

The solstices are the positions of the Earth's orbit that mark the longest and shortest days of the year. The winter solstice in the Northern Hemisphere is the shortest day, after which daylight hours grow longer. The summer solstice in the Northern Hemisphere falls on the longest day, after which daylight hours become shorter. The solstices can also be named for the month in which they occur. For example, the June solstice is the point in the Earth's orbit where the North Pole faces the sun. In the Northern Hemisphere, the June solstice is the longest day of the year. In the Southern Hemisphere, the June solstice is the shortest day of the year.

Position on Earth

Your place on the Earth relative to the equator also affects the number of daylight hours you get in a solar day. For example, during summer in the Northern Hemisphere, daylight hours increase the farther north you go; at this time, the Arctic gets very little nighttime darkness. In the winter, daytime is shorter the farther north you go. The seasonal changes in daylight hours are small near the equator and more extreme close to the poles.

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Benefits of darkness

What is naturally produced melatonin?

Melatonin, a hormone produced in the brain’s pineal gland, is responsible for the regulation of the body clock in each individual. Interestingly, the release of this hormone is largely controlled by exposure to natural light, or a lack thereof.
Melatonin production is first triggered in the evening, but the hormone continues to be released throughout the hours of darkness that follow (the conventional sleeping period). Levels of melatonin then drop with the breaking daylight and its production is suppressed until the next evening. Due to its dependence on a person being in a dark environment, melatonin is often referred to as the “hormone of darkness”.
It is precisely this link between darkness and melatonin, which informs advice to keep your bedroom dark and free from light-emitting electronics. Research has shown, for example, that both melatonin production and deep sleep phases are better maintained in the dark.

Melatonin and electric lighting

The introduction of electric light in the 19th century is often described as having had a negative influence on sleep. Before we had light bulbs and lamps in our homes, our circadian rhythms were dictated by natural light. People would wake with the first light of day and retire to bed early in the evening as the darkness fell.
The invention of artificial light however allowed people to make use of the evening after the sun went down. This change to our daily schedules is thought to have brought about changes in our sleep schedule, pushing our bedtime ever later.
Electric lighting continues to impact our sleep, not least because we are now free to work late into the evening and even throughout the night.

Melatonin sleep aids

Some research has suggested that melatonin supplements taken ahead of bedtime can help speed up the process of falling asleep and even improve sleep quality. There are limitations to this research however, having been conducted over a short time period. Thus far, this has prevented reliable conclusions being drawn about melatonin as a long-term solution for persistent poor sleep. Additionally, sleep is influenced by other factors apart from melatonin production and, as with many other prescription and over-the-counter sleep aids, the benefits of melatonin supplements do not persist once people stop taking them. In the UK and parts of Europe, melatonin is licensed for those over the age of 55 with chronic insomnia, for whom it has shown evidence of sleep improvement. Melatonin supplements are also commonly used as a “quick fix” by those who travel in an attempt to lessen the effects of jet lag. Further research is needed to establish whether melatonin supplements work any better than a placebo to minimize jet lag (Herxheimer and Petrie, 2002).

Do melatonin sleep aids have side effects?

Melatonin supplements are a relatively recent addition to the sleep aids market, having only gained popularity over the past decade or so. Research has therefore, only just begun to scratch the surface of the potential effects of melatonin, both negative and positive. It remains a future challenge therefore to answer questions such as when to take melatonin for sleep, or which melatonin dosage is best.
Melatonin is often seen as a ‘natural’ product when compared to prescription sleeping pills but this does not mean it escapes associated side-effects. So far, side-reported effects of melatonin include dizziness and headaches but long-term use of melatonin sleep aids has yet to be investigated.

Melatonin in older adults

Problems with sleep onset and maintenance in older adults are sometimes attributed to an age-related decrease in melatonin production. However, the results of research carried out to date has been mixed, meaning that further research is needed to establish whether age and melatonin production are indeed inversely proportional.

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Are there planets where it's always day/always night?

Have any planets been observed to rotate at the rate that would keep only one side facing their sun continuously? Do planets rotate at some mathematical constant related to their size and length of orbit around their sun?
This is a great question, planets rotating at the same rate as their star would have one side be permanently day and one side be permanently night as you have said. We have indeed found planets outside out solar system that work in this way. This effect is known as tidal locking and already happens with the moon where one side always faces the earth. (There is a small "wobble" with the moon where it does slightly change as it rotates about its own axis but the same side always faces the earth giving us a "dark side of the moon". You can see a great visualisation here : https://www.youtube.com/watch?v=vC7odtQHoPc ) In the solar system lots of moons are locked to their host planets.
If we imagine that we have a "host" body which might be a star or planet and an orbiting body around it. The main idea with tidal locking is that one half of the body that is orbiting its host is feeling more gravitational pull than the other causing a "bulge" on one side, facing the host. When the orbiting planet (or moon) is very close to its host this bulge will start to lead or lag the bodies orbit, depending on the system. This additional gravitational pull on one side serves to slow down the day-night rotation of the planet or moon until eventually it synchronises with the orbit around the host.
Only some bodies that are close enough are really susceptible to tidal locking. For it to really work you need to be close enough for the bulge to be able to be pulled off-centre the body orbits. Tidal locking also takes time to set in. When the planets and moons initially form they have their own rotation which over time decays. The time it takes to become locked depends on a few factors including the mass of the objects and the separation of the objects. This means that very close objects that are much lighter than their hosts lock quicker.
All in all tidally locked objects are close to their hosts. We have found some exoplanets that we believe are tidally locked. (For great examples try looking up hot Jupiters like Wasp-43b and HD209458b.)
I hope that answers your question, planets being tidally locked is definitely possible and we believe we have found very close in planets where, as you've said, it is permanently day on one side and permanently night on the other.

⦁ It is mustahabb to close the doors of houses at night

Question: Is this hadeeth saheeh? “At the time of sunset, we say Bismillaah and close the windows so that the jinn will not enter.”.
Answer
Praise be to Allaah.
It is narrated in a saheeh report that the Prophet (peace and blessings of Allaah be upon him) commanded us to close the doors and mention the name of Allaah at night, when night begins, when going to sleep. That is so that the Muslim may protect his house and his family from any harmful devil coming in, whether from among mankind or the jinn, and from harmful animals or insects coming in.
It was narrated from Jaabir ibn ‘Abd-Allaah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said:
“When the wings of the night spread – or when evening comes – keep your children in, for the devils come out at that time. Then when part of the night has passed, let them go. And close the doors and mention the name of Allaah, for the shaytaan does not open a closed door. And tie up your waterskins and mention the name of Allaah, and cover your vessels and mention the name of Allaah, even if you only put something over them, and extinguish your lamps.”
Narrated by al-Bukhaari (3280) and Muslim (2012). A version narrated by Muslim says:
“Cover vessels, tie up waterskins, close doors and extinguish lamps, for the shaytaan does not undo waterskins or open doors or uncover vessels.”
Imam al-Nawawi included this in a chapter entitled:
Chapter of the command to cover vessels, tie up waterskins, close doors and mention the name of Allaah over them, and extinguish lamps and fires when going to sleep, and keeping children and livestock in after sunset. End quote.
Muslim (2013) narrated in the same chapter that Jaabir ibn ‘Abd-Allaah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do not let your animals and children go out when the sun has set, until the first part of the night is over, for the devils come out when the sun sets, until the first part of the night is over.” Ibn Hibbaan narrated it in his Saheeh (4/90) as follows:
“Tie up your water skins and close your doors when you go to bed at night, and cover food and drink, for the shaytaan comes, and if he does not find the door closed he will enter, and if he does not find the waterskin tied up he will drink from it, and if he finds the door closed and the waterskin tied up, he does not open any water skin and he does not open a closed door, and if one of you cannot find something to cover his vessel in which is his drink, let him a place a stick over it.”
Imam Ibn Abd al-Barr (may Allah have mercy on him) said:
In this hadeeth there is a command to close doors of houses at night. This is a Sunnah which is enjoined to protect people against the devils of mankind and the jinn. As for the words “the shaytaan does not open a closed door and he does not untie a water skin” this is telling us of the blessing of Allaah, may He be glorified and exalted, to his human slaves, for he(the shaytaan) has not been given the power to open the door, or undo a water skin or uncover a vessel; these things have been forbidden to him, even though he has been given the power to do some things that are more difficult than that, such as going through and penetrating in ways that man cannot. End quote.
Al-Istidhkaar, 8/363
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
Ibn Daqeeq al-‘Eid said: in the command to close doors there are both religious and worldly benefits which protect lives and wealth from evildoers, and especially the devils.
As for the words “the shaytaan does not open a closed door,” this is an indication that the command to close doors serves the purpose of keeping the shaytaan away from mixing with people. The reason given is something that can be known through Revelation. The definite article is referring to the type, it does not mean any specific individual. End quote.
Fath al-Baari, 11/87.
He also said (may Allah have mercy on him):
Al-Qurtubi said: All commands on this issue are aimed at serving an interest for man, and are to be understood as recommendations, especially with regard to the one who does that with the intention of following the command.
Ibn al-‘Arabi said: Some people think that the command to close doors is general and is applicable to all times, but that is not the case. Rather it is limited to the night. It is as if the night is singled out in this case because the day is usually the time when people are awake, unlike the night. The whole idea is connected to the shaytaan, because he is the one who makes the mouse burn the house. End quote.
Fath al-Baari, 6/356 -- 357
Al-Khateeb al-Sharbeeni al-Shaafa’i (may Allaah have mercy on him) said:
When night comes, it is Sunnah to cover vessels even if only with a stick, to tie up water skins, and to close doors, mentioning the name of Allaah in all three cases, and to keep children and livestock in during the first part of the night, and to extinguish lamps when going to sleep. End quote.
Mughni al-Muhtaaj, 1/31
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
When going to sleep, one should close the door. End quote.
Sharh Riyaadh al-Saaliheen,.
See also the answer to question no. 125922.
And Allaah knows best.
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⦁ Staying up late.....a disease of modern times

They roam the streets when everyone is asleep. Or they may hang out with friends all night at the local ‘hookah place’. Or if they have nothing to do, some of them may even ‘egg’ some poor unsuspecting person’s house….’just because’. And if they are not ‘out there’, they are indoors, up until the wee hours of the morning, watching movies, listening to music, playing video games or chatting on the Internet, just because it’s ‘summer vacation’.
Yes, that’s right, brothers and sisters, that’s our youth during the summer.
And even when it is not summer, they stay up late even then. They think it is ‘cool’ to do so.
And, unfortunately, this ‘disease of late nights’ has become so widespread that we, as parents, don’t think much of it either. In fact, many of us adults are afflicted with this same disease ourselves. We come back from parties and dinners late at night and we let our kids do the same.
However what we don’t realize is….. that’s not how Allah Subhaanahu wa Ta’ala intended the order of things. In fact, it is quite contrary to the natural rhythm which Allaah has created in the universe and in mankind. Allaah says:
“It is He who has appointed the night a covering for you and sleep for a rest. The day He has appointed for rising.” (Surah Furqaan: 47)
Thus, the night is created for rest and the day for work and for seeking provision. Yet we do the exact opposite. We are up at night and sleep late into the day,
sometimes, up to or even beyond Dhuhr time, and waste much precious time. ‘Umar Radhi Allaahu Anhu used to punish people the people who did this and say: ‘You stay up for the first part of the night, then sleep for the last part of it?'”.
If we look at how the righteous before us spent their nights, we realize that they used their nights as a means to earning Jannah; praying, reciting the Qur’aan, crying and supplicating to Allaah. However, some of us Muslims, today take their nights as a means of entertainment and fun and even, aaoodhu billaah, a means of haraam and sin.
Actually, the Prophet (Sal Allaahu Alaiyhi wa Sallam) himself, did not like to stay up or even speak after Ishaa.
“The Prophet (Sal Allaahu Alaiyhi wa Sallam) used to like to delay the ‘Ishaa’ prayer and he hated to sleep before it or talk after it”. (Bukhaari).
He also disapproved of staying up late and warned against it.
He (Sal Allaahu Alaiyhi wa Sallam) said: “Beware of staying up after people’s movements settle.”
Why did he do that? Because staying up late might cause many harms. Some of them are:
1. It may cause one to totally miss the Fajr prayer
The Prophet (Sal Allaahu Alaiyhi wa Sallam) said:
“Between a man and kufr and shirk, there stands his giving up prayer.” (Muslim)
2. Even if you do wake up for Fajr, it is extremely hard to concentrate in the prayer or even know what one is saying.
3. Staying up late causes health problems, exhaustion, weakness, and loss of enthusiasm.
The Prophet (Sal Allaahu Alaiyhi wa Sallam) said:
“Staying up late is tiring and burdensome.” (Daarimi, Tabaraani, others)
4. It also leads to lack of responsibility and laziness, as it is extremely difficult to wake up early for school, job, etc. if you are up half the night. The person ends up sleeping late into the day, missing his work, and slacking in responsibility.
Al-Fudayl ibn ‘Iyaad said: There are two qualities that harden the heart: sleeping too much and eating too much.
5. We also miss out on the most blessed part of the day is the early morning. The Prophet (Sal Allaahu Alaiyhi wa sallam) said:
“The early morning has been blessed for my Ummah.”(Saheeh al-Jaami’).
That’s why whenever the Prophet (Sal Allaahu Alaiyhi wa sallam) sent out troops or an army, he would send them at the beginning of the day. But by sleeping late, we lose the blessings of this time and the profits we could have gained.
6. If we don’t sleep early, we can’t wake up in time for qiyaam ul-lail, a time for prayer, making dua and seeking forgiveness from Allaah, as the Prophet (Sal Allaahu Alaiyhi wa Sallam) said:
“Our Lord descends every night to the heavens when there is only one third of the night remaining and says: Who is supplicating to me so that I can respond to him? Who is asking Me for something so that I can grant him that which he is asking for? Who is seeking My forgiveness so that I can forgive him” (Bukhaari, Muslim) Is it befitting that we, as Muslims, miss this honorable and virtuous time and waste it either sleeping or following our desires? 7. Staying up late is also a major reason behind many of the moral crimes, community problems, car accidents, etc.
Exceptions to the rule:
The Ulama say that staying up late unnecessarily is nothing but a bad habit, unless it serves a purpose, such as praying and worshipping Allaah, seeking knowledge of Islaam, a person’s job, traveling, students preparing for exams, or other beneficial or permissible matters.
The Prophet (Sal Allaahu Alaiyhi wa Sallam) said: “Staying up late is for one of three categories of people; those traveling, those praying the night prayers, or those on their wedding night.” (Abu Ya’laa).
What Islaam encourages us to do is to sleep early, preferably right after Ishaa, then wake up early in time for our prayers (Fajr and Tahajjud, if possible) and then take advantage of the blessings of early morning for our work and provision. We are also encouraged to take a nap during the day, either before Dhuhr or after, if we are able to.
The Prophet (Sal Allaahu Alaiyhi wa Sallam) said:
“Take a nap, for the shayaateen do not take naps.” (al-Tabaraani–Saheeh).
Ishaaq ibn ‘Abd-Allaah said: “Taking a nap is one of the deeds of good people. It revitalizes the heart and helps one to pray qiyaam al-layl.”
Brothers and sisters, realize that staying up late is a diseased lifestyle that takes one away from the religion of Allaah and encourages the following of ones lusts and desires. One should avoid it unless there is a legitimate reason for it. Not only should we keep our kids away from it, we ourselves need to break this bad habit and develop healthy lifestyles and wholesome ways.
May Allaah enable us to realize the harms and evils of staying up late. May He enable us to utilize our time in the most productive manner, that is most pleasing to Him. Ameen.
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